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'[EE]: Driving an LED with +3V and a +5V logic PIN'
2002\10\15@134612 by Robert E. Griffith

Is there anything wrong with this?

            +3V ---.
                   L ANNODE
                   D CATHODE
+5V Logic output ---.

When the output is high, the LED will be reverse biased by around 2V, which
is within its spec (5V).

The reason I want to do this is that the Rabbit Core Module (uP + Memory +
Ethernet) I am using is a 3V device, but on board, it has a 5V out charge
pump for its Ethernet chip.  I am using that chip’s link status output to
drive an LED on my board which only has 3V availale.



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2002\10\16@031627 by Dwayne Reid

Should be just fine.  I have been doing this for years now on some of my
display cards with a whole bunch of LEDs on board - the LED drivers are
74hc595 shift registers powered from the 5V logic supply but the LEDs
operate from a dedicated 4.0V or 3.3V simple switcher SMPS.  Thousands of
cards in operation for several years with no LED failures.


At 01:43 PM 10/15/02 -0400, Robert E. Griffith wrote:

>When the output is high, the LED will be reverse biased by around 2V, which
>is within its spec (5V).

Dwayne Reid   <>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
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